WASHINGTON -(Dow Jones)- Leaders of a powerful House committee on Thursday sent a letter to Wyeth (WYE) asking the company to detail why a controversial ingredient in a drug used to treat heartworm in dogs is being tested in humans with river blindness in Africa.

Democratic leaders of the House Committee on Energy and Commerce are investigating why moxidectin, the active ingredient in ProHeart 6, is being tested in humans despite the drug having been pulled off the market in 2004 amid reports that 500 dogs died while taking it.

In June, the Food and Drug Administration said a reformulated version of ProHeart 6 could be allowed back in the U.S. to treat heartworm in dogs, though distribution of the product would be restricted. Heartworm is a parasite that affects millions of dogs.

Sen. Chuck Grassley, R.-Iowa, sent a letter to the FDA in June questioning why the drug was allowed back in the U.S.

According to a letter sent Thursday by Rep. John Dingell, D.-Mich., who chairs the energy and commerce committee, and Rep. Bart Stupak, D.-Mich., Wyeth is testing moxidectin in people infected with a parasite that causes river blindness, a major cause of blindness in West Africa.

Wyeth spokesman Doug Petkus said the company is reviewing the letter and will respond to the committee. He said about a decade ago the World Health Organization approached the company to see whether moxidectin could help kill the worms that transmit river blindness between people.

The current treatment for river blindness, Merck & Co.'s (MRK) Stromectol, isn't effective at killing those worms, Petkus said. Petkus said studies show moxidectin is as safe as Merck's drug, and is effective at killing the large worms.

He added that the WHO wants to begin Phase III trials in a few years with about 1,500 people from two African nations.

The trials are being conducted in collaboration with the Special Programme for Research and Training in Tropical Diseases, which is sponsored by the United Nations Children's Fund, United Nations Development Programme, World Bank, and the WHO, according to Thursday's letter.

Petkus said since the company reintroduced ProHeart 6 into the market in June, the safety profile of the drug has "continued to be positive."

-By Jared A. Favole, Dow Jones Newswires; 202.862.9207; jared.favole@ dowjones.com

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  07-31-08 1839ET
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